The Raphnet Wii Classic Controller to USB is the real del. It's seriously as good as a Wired Xbox 360 guitar. Legitimately buy one of these if you have a Wii Guitar.
Wii Guitar / Classic Controller to USB Adapter for Clone Hero – DDRPad.com
Triple Controller - Classic Gaming USB Adapter V2 from Timville on Tindie
Tribute64 V2 2.4G Wireless Controller for N64/Switch/USB - Red
Amazon.com: May Flash NES/SNES Controller Adapter for NES Classic and Wii U/Wii : Video Games
PlayStation Controller to USB Adapter – DDRPad.com
Wii Guitar / Classic Controller to USB Adapter for Clone Hero – DDRPad.com
raphnet. - Classic controller to USB adapter - V2
raphnet. - Classic controller to USB adapter - V2
raphnet Classic Controller to USB Adapter Wii Guitar Drum Turntable NES SNES V3 | eBay
raphnet. - Dual Classic controller to USB adapter - V2
Classic controller to Gamecube/Wii adapter – Performance Gaming